Structural and magnetic properties of Zn1−xSbxCr2−x/3Se4 (x=0.11, 0.16 and 0.20) single crystals

Single crystals of Zn_1−xSb_xCr_2−x/3Se₄ based on the ZnCr₂Se₄ spinel, which is known to exhibit interesting magnetic and electronic transport properties, have been prepared by solid state reaction from the appropriate selenides. Three compounds of different Sb content (x=0.11, 0.16, and 0.20) were studied by X-ray diffraction, X-ray photoelectron scattering technique and macroscopic magnetic measurements with the aim to determine (i) stability of the cubic symmetry and (ii) influence of the Sb admixture on the magnetic properties. The results show that the Sb³⁺ and Zn²⁺ ions share the tetrahedral sites in the spinel structure, while the Cr³⁺ions carrying magnetic moments, are located in the octahedral sites. The X-ray photoelectron spectroscopy (XPS) data indicate that in this series of compounds the chromium ions have a 3d³ electronic configuration. The three samples studied order antiferromagnetically at low temperatures, with the magnetic characteristics being hardly altered with respect to those reported for the parent ZnCr₂Se₄ compound.     

Magnetocaloric effect in the ternary DyCo3B2 compound

A large magnetocaloric (MCE) effect has been observed for the ternary compound DyCo₃B₂. This material shows the magnetic ordering below T_C = 22 K for H = 0 T. MCE has been determined based on the isothermal magnetization curves measurements and the isomagnetic heat capacity dependence on temperature. The maximum magnetic entropy change −ΔS_M = 17.5 J kg⁻¹K⁻¹ and the adiabatic temperature change ΔT_ad = 14 K have been observed in the neighborhood of the magnetic phase transition at the magnetic field change of 9 T. The analysis of the magnetic contribution to the specific heat indicates on the important role of the crystal electric field and the anisotropy for the properties of the DyCo₃B₂ compound.     

Magnetic properties of Fe2P-type Tb6FeTe2, Tb6CoTe2, Tb6NiTe2 and Er6FeTe2 compounds

The magnetic ordering of the Fe₂P-type Tb₆FeTe₂, Tb₆CoTe₂ Tb₆NiTe₂ and Er₆FeTe₂ phases (space group P6¯2m) has been investigated through magnetization measurement and neutron powder diffraction. Tb₆FeTe₂, Tb₆CoTe₂ and Tb₆NiTe₂ demonstrate high-temperature ferromagnetic and low-temperature spin reorientation transitions, whereas Er₆FeTe₂ shows antiferromagnetic transition, only.The Tb₆FeTe₂ and Tb₆NiTe₂ phases show same high-temperature collinear ferromagnetic structure, whereas Tb₆FeTe₂ is the commensurate non-collinear ferromagnet and Tb₆NiTe₂ is the canted ferromagnetic cone with K₁=[0, 0, ±3/10] and K₂=[±2/9, ±2/9, 0] wave vectors at 2 K. The magnetic structure of Er₆FeTe₂ is a flat spiral with K₁=[0, 0, ±1/10] at 2 K. The magnetic entropy change for Tb₆NiTe₂ is ΔS_m=−4.86 J/kg K at 229 K for the field change Δμ₀H=0-5 T.In addition, novel Fe₂P-type Gd₆FeTe₂, Zr₆FeTe₂, Hf₆FeTe₂, Dy₆NiTe₂, Zr₆NiTe₂ and Hf₆NiTe₂ phases have been obtained.     

Vanadium(II) alkyls. Synthesis and X-ray crystal structures of trans-VMe2(dmpe)2 and cis-V(CH2SiMe3) 2(dmpe)2

Treatment of the vanadium(II) tetrahydroborate complex trans-V(η¹-BH₄)₂(dmpe)₂ with (trimethylsilyl) methyllithium gives the new vanadium(II) alkyl cis-V(CH₂SiMe₃)₂(dmpe)₂, where dmpe is the chelating diphosphine 1,2-bis(dimethylphosphino)ethane. Interestingly, this complex could not be prepared from the chloride starting material VCl₂(dmpe)₂. The CH₂SiMe₃ complex has a magnetic moment of 3.8 μ_B, and has been characterized by ¹H NMR and EPR spectroscopy. The cis geometry of the CH₂SiMe₃ complex is somewhat unexpected, but in fact the structure can be rationalized on steric grounds. The X-ray crystal structure of cis-V(CH₂SiMe₃)₂(dmpe)₂ is described along with that of the related vanadium(II) alkyl complex trans-VMe₂(dmpe)₂. Comparisons of the bond distances and angles for VMe₂(dmpe) ₂, V---C = 2.310(5) Å, V---P = 2.455(5) Å, and P---V---P = 83.5(2)° with those of V(CH₂SiMe₃)₂(dmpe)₂, V---C = 2.253(3) Å, V---P = 2.551(1) Å, and P ---V---P = 79.37(3)° show differences due to the differing trans influences of alkyl and phosphine ligands, and due to steric crowding in latter molecule. The V---P bond distances also suggest that metal-phosphorus π-back bonding is important in these early transition metal systems. Crystal data for VMe₂(dmpe)₂ at 25°C: space group P2₁/n, with a = 9.041(1) Å, b = 12.815(2) Å, c = 9.905(2) Å, β = 93.20(1)°, V = 1145.8(5) ų, Z = 2, R_F = 0.106, and R_wF =0.127 for 74 variables and 728 data for which I 2.58 σ(I); crystal data for V(CH₂SiMe₃)₂(dmpe)₂ at −75°C: space group C2/c, with a = 9.652(4) Å, b = 17.958(5) Å, c = 18.524(4) Å, β = 102.07(3)°, V= 3140(3) ų, Z = 4, R_F = 0.033, and R_wF = 0.032 for 231 variables and 1946 data for which I 2.58 σ(I).     

Magnetic properties of Fe2P-type R6CoTe2 compounds (R=Gd-Er)

The magnetic structure of the Fe₂P-type R₆CoTe₂ phases (R=Gd-Er, space group P6¯2m) has been investigated through magnetization measurement and neutron powder diffraction. All phases demonstrate high-temperature ferromagnetic and low-temperature transitions: T_C=220 K and T_CN=180 K for Gd₆CoTe₂, T_C=174 K and T_CN=52 K for Tb₆CoTe₂, T_C=125 K and T_CN=26 K for Dy₆CoTe₂, T_CN=60 K and T_N=22 K for Ho₆CoTe₂ and T_CN∼30 K and T_N∼14 K for Er₆CoTe₂.Between 174 and 52 K Tb₆CoTe₂ has a collinear magnetic structure with K₀=[0, 0, 0] and with magnetic moments along the c-axis, whereas below 52 K it adopts a non-collinear ferromagnetic one.Below 60 K the magnetic structure of Ho₆CoTe₂ is that of a non-collinear ferromagnet. The holmium magnetic components with a K₀=[0, 0, 0] wave vector are aligned ferromagneticaly along the c-axis, whereas the magnetic component with a K₁=[1/2, 1/2, 0] wave vector are arranged in the ab plane. The low-temperature magnetic transition at ∼22 K coincides with the reorientation of the Ho magnetic component with the K₀ vector from the collinear to the non-collinear state.Below 30 K Er₆CoTe₂ shows an amplitude-modulate magnetic structure with a collinear arrangement of magnetic components with K₀=[0, 0, 0] and K₁=[1/2, 1/2, 0]. The low-temperature magnetic transition at ∼14 K corresponds to the variation in the magnitudes of the M_Er^K0 and M_Er^K1 magnetic components.In these phases, no local moment was detected on the cobalt site.The magnetic entropy of Gd₆CoTe₂ increases from ΔS_mag=−4.5 J/kg K at 220 K up to ΔS_mag=−6.5 J/kg K at 180 K for the field change Δμ₀H=0-5 T.     

Crystal structure, thermal and magnetic properties of Cr2V4O13

Cr₂V₄O₁₃, a tetravanadate of Cr³⁺ has been prepared by repeated heating of stoichiometric amounts of Cr₂O₃ and V₂O₅ and its crystal structure is refined by Rietveld refinement of the powder XRD data. This compound crystallizes in a monoclinic lattice with unit cell parameters: a=8.2651(3), b=9.2997(3), c=14.5215(5) Å, β=102.618(3)°, V=1089.21(6) Å³ and Z=4 (Space group: P2₁/c). The U shaped (V₄O₁₃)⁶⁻ formed by corner connected VO₄ tetrahedra links the Cr₂O₁₀ (dimers of two edge shared CrO₆ octahedra) forming a three dimensional network structure of Cr₂V₄O₁₃. This compound is stable up to 635 °C and peritectically decomposes to orthorhombic CrVO₄ and V₂O₅ above this temperature. A possible long range antiferromagnetic ordering below 10 K is suggested from the squid magnetometry and electron paramagnetic resonance (epr) spectroscopic studies of Cr₂V₄O₁₃.     

Magnetic and thermodynamic properties of NdT2Ge2 (T=Pd, Ag) compounds

Physical properties of NdPd₂Ge₂ and NdAg₂Ge₂, crystallizing with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric, electrical transport as well as by neutron diffraction measurements. The specific heat studies and neutron diffraction measurements were performed down to 0.30 K and 0.47 K, respectively. Both compounds exhibit antiferromagnetic ordering below T_N equal to 1.5 K for NdPd₂Ge₂ and 1.8 K for NdAg₂Ge₂. Neutron diffraction data for the latter germanide indicate antiferromagnetic collinear structure described by the propagation vector k=(0.5, 0, 0.5). The Nd magnetic moments equal to 2.24(5) μ_B at 0.47 K are aligned along the a-axis and have the +− sequence within the crystal unit cell. For NdPd₂Ge₂ only very small Bragg peaks of magnetic origin were observed in the neutron diffraction patterns measured below T_N, thus hampering determination of the magnetic structure. Both compounds exhibit metallic-like electrical conduction. From the specific heat data the crystal electric field (CEF) levels schemes were determined. Difference between the overall CEF splitting in the two compounds is correlated with their structural parameters.     

Crystal structure, magnetic susceptibility and Mössbauer spectroscopy of the mixed-valence iron phosphate Na1/2Cu4/3Fe2(PO4)3

Single crystals of a new mixed-valent iron phosphate Na_1/2Cu_4/3Fe₂(PO₄)₃ have been synthesized by a flux method and structurally characterized from X-ray diffraction data. Crystal data: space group ; ; ; ; α=105.881(1)°; β=107.202(1)°; γ=101.467(1)°; Z=2; R₁=0.03; wR₂=0.093. The three-dimensional structure was found to be closely related to that of the well known Howardevansite structural type. It results from infinite chains of CuO₅ and FeO₆ polyhedra, joined together by (Cu,□)O₆ octahedra and PO₄ tetrahedra by corner-sharing. The large cavities in framework are occupied by Na⁺ ions. The magnetic susceptibility study revealed an antiferromagnetic behavior with Neel temperature of approximately 40 K. The Mössbauer spectroscopy confirmed the presence of iron in both +2 and +3 oxidation states.     

The crystallographic and magnetic characteristics of Sr2CrO4 (K2NiF4-type) and Sr10(CrO4)6F2 (apatite-type)

Solid-state reaction between SrCO₃, Cr₂O₃ and SrF₂ has produced the apatite phase Sr₁₀(CrO₄)₆F₂ and Sr₂CrO₄ which adopts the K₂NiF₄-type structure. The reaction outcome was very sensitive to the heating rate with rapid rise times favouring the formation of Sr₂CrO₄, which has been synthesised at ambient pressure for the first time. Powder X-ray diffraction and electron diffraction confirmed that Sr₂CrO₄ adopts a body centred tetragonal cell (space group I4/mmm) with lattice parameters a=3.8357(1) Å and c=12.7169(1) Å, while a combination of neutron and X-ray diffraction verified Sr₁₀(CrO₄)₆F₂ is hexagonal (space group P6₃/m) with lattice parameters a=9.9570(1) Å and c=7.4292(1) Å. X-ray photoelectron spectroscopy and magnetic measurements were used to characterise the oxidation states of chromium contained within these phases.     

Electron spin resonance of diluted solid solutions of Gd2O3 in CeO2

Electron spin resonance spectra of Gd³⁺ in diluted solid solutions of Gd₂O₃ in CeO₂ have been studied at room temperature for Gd concentrations between 0.01 and 1.00 mol%. While in the case of Mn²⁺:CeO₂ samples, both the linewidth and the line intensity go through a maximum between 0.2 and 0.4% Mn and then start to decrease, in the case of Gd³⁺:CeO₂ samples the linewidth and the line intensity increase monotonically with the dopant concentration. This as taken as evidence that in Gd₂O₃-CeO₂ diluted solid solutions there are no clustering effects similar to the ones observed in Mn:CeO₂ solid solutions. It is not clear why clustering effects are present in Mn:CeO₂ solid solutions and not in Gd:CeO₂ solid solutions; however, it seems reasonable to assume that this is due to the fact that the ionic radius of Mn²⁺ (81 pm) is about 25% smaller that that of Gd³⁺ (107.8 pm). In any case, the fact that Gd:CeO₂ solid solutions do not exhibit clustering effects means that ESR linewidth data can be used to estimate the concentration of Gd in CeO₂ samples, as it is possible to do in several solid solutions of paramagnetic ions in ceramic materials. The results also suggest that the range of the exchange interaction between Gd³⁺ ions in CeO₂ is about 0.89 nm.     

Structural study and physical properties of a new phosphate KCuFe(PO4)2

Single crystals of a new phosphate KCuFe(PO₄)₂ have been prepared by the flux method and its structural and physical properties have been investigated. This compound crystallizes in the monoclinic system with the space group P2₁/n and its parameters are: a=7.958(3) Å, b=9.931(2) Å, c=9.039(2) Å, β=115.59(3)° and Z=4. Its structure consists of FeO₆ octahedra sharing corners with Cu₂O₈ units of edge-sharing CuO₅ polyhedra to form undulating chains extending infinitely along the b-axis. These chains are connected by the phosphate tetrahedra giving rise to a 3D framework with six-sided tunnels parallel to the [101] direction, where the K⁺ ions are located. The Mössbauer spectroscopy results confirm the exclusive presence of octahedral Fe³⁺ ions. The magnetic measurements show the compound to be antiferromagnetic with C_m=5.71 emu K/mol and θ=−156.5 K. The derived experimental effective moment μ_ex=6.76μ_B is somewhat higher than the theoretical one of μ_th=6.16μ_B, calculated taking only into account the spin contribution for Fe³⁺ and Cu²⁺ cations. Electrical measurements allow us to obtain the activation energy (1.22 eV) and the conductivity measurements suggest that the charge carriers through the structure are the potassium cations.     

Large magnetoresistance in a ferromagnetic cobaltite: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba_0.9CoO_2.6 crystallizes in the 12H structure [P6₃/mmc, a=5.6612 (1) Å and c=28.4627(8) Å]. Interestingly, this compound is a ferromagnet with a Curie temperature T_C=50 K and a saturation magnetization . This value is smaller than expected from the effective paramagnetic moment, μ_eff=3.7μ_B/Co, corresponding to an average spin per Co, from which one would expect . This suggests either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoO_n polyhedra. A clear transition in the electrical resistivity is found at T_C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spin-charge coupling is evidenced by the large negative magnetoresistance effect optimum near T_C=50 K, with .     

Synthesis, structure and characterisation of Fe0.50Ti2(PO4)3: A new material with Nasicon-like structure

A new iron titanyl phosphate Fe_0.50Ti₂(PO₄)₃ was synthesized by both solid-state reaction and Cu²⁺-Fe²⁺ ion exchange method. The material was then characterized by X-ray diffraction, Mössbauer, magnetic susceptibility measurements and optical absorption. The crystal structure of the compound was refined, using X-ray powder diffraction data, by the Rietveld profile method; it crystallizes in the rhombohedral system, space group , with a=8.511(1) Å and c=20.985(3) Å, V=1316.45(3) Å³ and Z=6. The structure, which is compared to that of Mn_0.50Ti₂(PO₄)₃ is built up from [TiO₆] octahedra and [PO₄] tetrahedra which are linked by corner sharing along the c-axis. Fe²⁺ cations are located in half of the antiprism M_I sites and are orderly distributed with vacancies within the two possible positions of the M_I sites of . These results were supported by the Mössbauer studies that showed the presence of one Fe²⁺ site in the high spin state (t_2g⁴e_g²). The Curie-Weiss-type behavior is observed in the magnetic susceptibility. Diffuse reflectance spectrum indicates the presence of octahedrally coordinated Fe²⁺ ions.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.     

Synthesis and crystal structure of the palladium oxides NaPd3O4, Na2PdO3 and K3Pd2O4

NaPd₃O₄, Na₂PdO₃ and K₃Pd₂O₄ have been prepared by solid-state reaction of Na₂O₂ or KO₂ and PdO in sealed silica tubes. Crystal structures of the synthesized phases were refined by the Rietveld method from X-ray powder diffraction data. NaPd₃O₄ (space group Pm3¯n, a=5.64979(6) Å, Z=2) is isostructural to NaPt₃O₄. It consists of NaO₈ cubes and PdO₄ squares, corner linked into a three-dimensional framework where the planes of neighboring PdO₄ squares are perpendicular to each other. Na₂PdO₃ (space group C2/c, a=5.3857(1) Å, b=9.3297(1) Å, c=10.8136(2) Å, β=99.437(2)°, Z=8) belongs to the Li₂RuO₃-structure type, being the layered variant of the NaCl structure, where the layers of octahedral interstices filled with Na⁺ and Pd⁴⁺ cations alternate with Na₃ layers along the c-axis. Na₂PdO₃ exhibits a stacking disorder, detected by electron diffraction and Rietveld refinement. K₃Pd₂O₄, prepared for the first time, crystallizes in the orthorhombic space group Cmcm (a=6.1751(6) Å, b=9.1772(12) Å, c=11.3402(12) Å, Z=4). Its structure is composed of planar PdO₄ units connected via common edges to form parallel staggered PdO₂ strips, where potassium atoms are located between them. Magnetic susceptibility measurements of K₃Pd₂O₄ reveal a Curie-Weiss behavior in the temperature range above 80 K.     

Mechanism of the low thermal expansion in α-Hf2O(PO4)2 and its zirconium analog

The thermal expansion of recently characterized α-Zr₂O(PO₄)₂ and α-Hf₂O(PO₄)₂ is found to be very low (respectively 2.6 and 2.9×10⁻⁶ K⁻¹, 20-900  °C). High-temperature X-ray diffraction and Rietveld analysis allowed to identify a dual contraction mechanism, involving a classical ring deformation and the rocking of bridging oxygens.     

Synthesis, structure determination, and infrared spectroscopy of (NpO2)2(SO4)(H2O)4: Prevalence of cation-cation interactions and cationic nets in neptunyl sulfate compounds

The compound (NpO₂)₂(SO₄)(H₂O)₄ was synthesized by evaporation of a Np⁵⁺ sulfate solution. The crystal structure was determined using single crystal X-ray diffraction and refined to an R₁=0.0310. (NpO₂)₂(SO₄)(H₂O)₄ crystallizes in triclinic space group P-1, a=8.1102(7) Å, b=8.7506(7) Å, c=16.234(1) Å, α=90.242(2)°, β=92.855(2)°, γ=113.067(2)°, V=1058.3(2) Å³, and Z=2. The structure contains neptunyl pentagonal bipyramids that share vertices through cation-cation interactions to form a sheet or cationic net. The sheet is decorated on each side by vertex sharing with sulfate tetrahedra, and adjacent sheets are linked together through hydrogen bonding. A graphical representation of (NpO₂)₂(SO₄)(H₂O)₄ was constructed to facilitate the structural comparison to similar Np⁵⁺ compounds. The prevalence of the cationic nets in neptunyl sulfate compounds related to the overall stability of the structure is also discussed.     

Analysis of (NH4)2SO4/(NH4)H2PO4 mixtures by thermogravimetry and X-ray diffraction

(NH₄)₂SO₄ and (NH₄)H₂PO₄ are the principal components in the powder material used in fire extinguishers. In this paper the mutual influence in their thermal decomposition is investigated by thermogravimetry. Two methods for the quantification of both salts in mixtures (NH₄)₂SO₄/(NH₄)H₂PO₄ are proposed. The first employs thermogravimetry and is based on the measurement of the mass fraction in the 500-550 °C interval, once (NH₄)₂SO₄ has totally decomposed to yield gaseous products. The second uses some selected peaks in the X-ray diffractogram.     

Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1−x)1−zO6

Perovskite-type cobaltates in the system La₂Co_1+z(Mg_xTi_1−x)_1−zO₆ were studied for z=0≤x≤0.6 and 0≤x<0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption near-edge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350 °C. The space group symmetry of the structure changes from P2₁/n via Pbnm to R3¯c with both increasing Mg content and increasing Co content. The La₂Co(Mg_xTi_1−x)O₆ (z=0) compounds show anti-ferromagnetic couplings of the magnetic moments for the Co below 15 K for x=0, 0.1 and 0.2. XANES spectra show for the compositions 0≤x≤0.5 a linear decrease in the L₃/(L₃+L₂) Co-L_2,3 edge branching ratio with x, in agreement with a decrease of the average Co ion spin-state, from a high-spin to a lower-spin-state, with decreasing nominal Co²⁺ ion content.